The invention is in regard to a process for measuring liquid levels as well as to a liquid level sensor with a number of capacitive sensors arranged side-by-side along the filling range of a tank, and consisting of field electrodes and measuring electrodes located diametrically opposite to each other.
Liquid level measuring sensors are particularly useful in gas tanks as, for example, in gas tanks of motor vehicles and motorcycles. For safety reasons, gas tanks are frequently given an irregular shape, which limits the use of traditional potentiometer sensors. The resolution and accuracy of potentiometer sensors in residual.quantities or in full tanks have limitations. In addition, potentiometer sensors are susceptible to wear and tear and may break down for mechanical reasons.
Also known, as an alternative to potentiometer sensors, are the capacitive sensors. For example, DE-AS 22 21 741 and DE-PS 25 15 065 describe liquid level sensors with a bank of capacitors positioned one above the other along a filling range of a tank to which an impulse voltage is applied at the capacitors"" field electrodes. The measuring electrodes are connected to a differential amplifier. The dielectric constant of the medium in which the liquid level sensor is submerged influences the capacitor""s capacitance. In DE-AS 22 21 741, two neighboring capacitors of the same capacitance form one differential capacitor. To the differential capacitors, impulse signals are injected at their common electrodes, and the differential amplifier""s exit signal equals zero if both capacitors belonging to one and the same differential capacitor are located in the same medium. Correspondingly, the differential signal is other than zero when the two capacitors are located in different media. The differential amplifier""s exit signal Is connected to an impulse counter and the number of the impulses other than zero becomes the measurement of the liquid level.
This arrangement has the disadvantage that the two capacitors forming one differential capacitor must possess the exact same capacitance. DE-OS 49 37 927 describes an arrangement for measuring liquid levels in which the capacitors are combined first into one group, and then into at least one further group. Each group contains two sub-groups of parallel-wired capacitors. Two capacitors of the same group positioned next to each other along the direction of the filling range have approximately the same capacities if the dielectric constant is the same. The sub-groups belonging to the same group are each connected to a common comparator unit, which forms a digital comparison signal in relation to the difference of the resulting capacities of the sub-groups. The reference value for the resulting capacity of one sub-group is the resulting capacity of another sub-group. In that manner, same-direction aging and environmental influences can be compensated for.
The disadvantage of this arrangement is that the capacitors must have the exact same capacities. In addition, problems arise from parasitical stray capacitance because each impulse addresses an entire row of capacitors, which influence mutually each other.
In DE-PS 31 14 678 a liquid level indicator is described, in which several measuring electrodes are combined section ally and connected to an evaluation circuit. The sensor elements have one single back plate. The capacitors located below the level of a liquid are wired parallel by the liquid""s dielectric and the resulting capacity becomes the measure for the liquid""s level. Therefore, a reference value is necessary to convert the measured capacity into a proportional liquid level. The reference value is dependent on the medium that is to be measured and must be newly calibrated each time the container is filled. In addition, stray capacitance occurs between the back plate and the measuring electrodes and between the measuring electrodes themselves.
The DE-OS 39 26 218 A1 shows a liquid level measuring arrangement in which the level can be determined in homogeneously distributed media without a reference value. To accomplish this, the measuring electrodes are connected.
Individually to a comparison unit and evaluated cyclically and sequentially. In doing so, each value is compared in a comparator to a reference value, which is the same for capacitors of the same size. There is only one back plate involved and the container wall can be used for this back plate.
In DE-PS 196 44 777 C1 a liquid level sensor is described in which the measuring electrodes of the capacitive sensors are connected to a liquid level evaluation circuit in a way that allows them to be triggered individually. In each field, the back plates of a respective group of capacitive sensors are interconnected with one field electrode, and the field electrodes of each of the fields are connected with the liquid level evaluation circuit in an individually selective manner. By subdividing the usually single back plates into a number of field electrodes, the problem of stray capacitance is thus reduced.
Frequently, water is found on the bottom of the tank that has collected there. In addition, the medium""s dielectric constant may vary. This may cause measurement errors with the above-mentioned liquid level sensors.
The problem of the invention was, therefore, to provide an improved process for measuring liquid levels, with a number of capacitive sensors arranged side-by-side along a filling range as well as a liquid level sensor by which the level can be determined in a simple and reliable manner, even when the tank is used for unknown or varying media.
The following steps solve the problem of the invention:
An impulse current generator for a field electrode with a measuring signal and a neighboring field electrode with a phase-displacement measuring signal,
Measurement of a resulting signal at the individual measuring electrodes,
Determination of the resulting signal""s phase displacement in relation to a reference signal, and
Determination of the level in relation to the phase displacement.
When, for example, two measuring signals of opposing phases are simultaneously applied to the capacitive sensors, the signals cancel each other out when the sensors in question are positioned in the same medium. It is advantageous, in this respect, that the measuring signals are in phase opposition, that is, in a 180xc2x0 phase displacement. In addition, a signal is always applied to two sensors. As long as the sensors are positioned in a medium with a dielectric constant of E less than  greater than 1, the two signals in the phase become compensatoryxe2x80x94even if the media vary.
When one of the sensors is in air and the dielectric constant E=1, a phase displacement occurs in the compound signal which is made up of the sum of the two phase-opposed measuring signals. The level height can be determined by the phase-displacement.
In order to avoid parasitic stray capacitances, it is advantageous to connect the sensors, to which no measuring signal is being applied, to a mass potential. For this purpose, each of the measuring electrodes can have a change-over switch, by means of which it can be connected either to a measure signal, to the phase-opposed measuring signal, or to the mass potential. The switches can be controlled by a microprocessor, for example.
It is advantageous to apply impulse signals to the sensors along the filling range sequentially and continuously, so that the filling range is scanned continuously.
The resulting signals at the sensors"" measuring electrodes are advantageously amplified by a summing amplifier and led to a phase comparator for measuring the phase displacement.
In addition, it is particularly advantageous to determine the amplitude of the compound signal. By comparing it to the reference signal, it is possible to determine the medium surrounding the respective measuring electrode.
In order to avoid parasitical stray capacitance, it is also advantageous to connect the measuring electrodes selectively to the summing amplifier or a mass potential through a switch. These switches can again be controlled by a microprocessor.
It has been found to be of particular-advantage to provide for a great number of field electrodes to which measuring signals can be applied respectively. The resulting signal should be measured at the measuring electrodes whereby several field electrodes are provided for each measuring electrode. It is particularly advantageous if four field electrodes each work together with one measuring electrode.